I can guess that, but I was hoping you could tell me which one, since most of them would exit that jar pretty quick once the cap war removed to allow for the match to be put in. I assumed it was a special property of a specific gas of some sort.

But it it's liquid then why does the substance still burn at the top once it has proceeded to burn at the bottom as well? That indicates gas, but, as dragonette said, it has to be heavier than oxygen, highly flammable and stable enough to hold it in front of a webcam in a large jug without fear of it exploding when you drop a match in.

Hmmm.....Fire needs oxygen to burn, while the hydrogen might have still been at near liquid tempurature, its still not a liquid. It had liquid properties while it was burning, but it was still lighter than air. hmmm.... The air inside the jug pushes the hydrogen down and the hydrogen is trying to find the one place it can get out of the way of the heavier air which is out of the jug and through the top where the flame is . The match wasnt dropped in, It would have sunk STRAIGHT to the bottom, lighting ALL of the gas, thus making the jug explode. Im not Bill Nye, just a high school kid in advanced chemestry.